Tapered block copolymers of monovinylarenes and conjugated dienes

Abstract: A method for preparing tapered block copolymers which are particularly useful for blend components in blends with polymers of styrene. In the first embodiment of this invention the copolymers are prepared in a polymerization process by sequentially charging: (1) an initiator and monovinylaromatic monomers in the presence of a randomizer; (2) an initiator and monovinylaromatic monomers; (3) a mixture of monovinylaromatic and conjugated diene monomers; and (4) a coupling agent; to produce a polymodal tapered block copolymer. In a second embodiment of the invention tapered block copolymers are prepared in a polymerization process by sequentially charging: (1) an initiator and monovinylaromatic monomers in the presence of a randomizer; (2) an initiator and monovinylaromatic monomers; (3) a mixture of monovinylaromatic and conjugated diene monomers; (4) a mixture of monovinylaromatic and conjugated diene monomers; and (5) a coupling agent; to produce a polymodal tapered block copolymer. The invention copolymers and blends of the invention copolymers with polymers of styrene are particularly useful for applications such as packaging and food or drink containers which require transparency, low blueness, colorlessness, good impact strength and ductility.

Description: Field of the Invention This invention relates to tapered block copolymers of monovinylarenes and conjugated dienes and methods of preparation of these tapered block copolymers. Background of the Invention There has developed in the polymer field, and especially in the packaging and related industries, a need for thermoplastic polymers that can be formed into colorless, transparent articles having good impact strength and ductility There are needs for polymers which are useful as single components as well as for use in blends with other commonly used polymers to make articles with improved properties. The polymers satisfying these needs should be suitable for use with conventional extrusion, injection and blow molding equipment and also suitable for use in other methods of forming plastics into containers, tubes, films, fibers, and the like. Polystyrene, high impact polystyrene, branched block copolymers, and the like have been developed to meet these criteria with various degrees of satisfaction. Much effort has been directed to the preparation of substantially transparent block copolymer resins with a variety of block structures produced by a variety of monomer addition sequences and a variety of coupling agents. Desirable properties and an economic advantage can be obtained by blending some monovinylaromatic-conjugated diene copolymers with polystyrene polymers. However, because blueness of blends of monovinylaromatic-conjugated diene copolymers with polystyrene polymers cannot be predicted by rule of mixtures behaviors, getting a desirable combination of properties can be a complicated task. Sometimes relatively colorless monovinylaromatic-conjugated diene copolymers give blends with high blueness when blended with colorless general purpose polystyrene. Exemplary publications in this field are US-A-4 167 545 and EP-A-0 512 530. Blueness of articles formed from various copolymers and blends of copolymers with other polymers is a longstanding problem in applications where colorless materials which also have good impact strength and ductility are desirable. Specific examples include materials for water and food containers. Summary of the Invention It is an object of this invention to provide resinous tapered block copolymers of vinyl-substituted aromatic hydrocarbons and conjugated dienes from which can be made articles with low blueness and acceptable impact strength and ductility. It is another object of this invention to provide resinous tapered block copolymers of vinyl-substituted aromatic hydrocarbons and conjugated dienes suitable for use in blends with polymers of styrene. particularly blends from which can be made articles that exhibit low blueness and acceptable impact strength and/or ductility. Further, it is an object to provide resinous tapered block copolymers of vinyl-substituted aromatic hydrocarbons and conjugated dienes which can be used in smaller amounts than some of the other commonly used resinous polymodal monovinyl substituted aromatic-conjugated diene block copolymers in blends with polymers of styrene to achieve similarly low blueness levels and good impact strength and/or ductility in articles made from the blends. A further object of this invention is to provide novel processes for making resinous tapered block monovinylaromatic/conjugated diene copolymers, including copolymers suitable for use in blends. According to the invention, copolymers are prepared under solution polymerization conditions in a reaction zone by: (a) charging a monovinylaromatic monomer and an initiator in the presence of a randomizer, which is an ether, a thioether, or a tertiary amine, and allowing polymerization to occur until essentially no free monomer is present; thereafter (b) charging an initiator and a monovinylaromatic monomer and allowing polymerization to occur until essentially no free monomer is present; thereafter (c) charging a mixture of a monovinylaromatic monomer and a conjugated diene monomer and allowing polymerization to occur until essentially no free monomer is present; and thereafter (d) charging a mixture of a monovinylaromatic monomer and a conjugated diene monomer; and thereafter (e) charging the reaction mixture with a coupling agent, wherein in steps (a) and (b) 0.005-1.5 parts by weight per 100 parts by weight of total monomers each of initiator are employed, wherein each said monovinylaromatic monomer contains 8 to 12 carbon atoms and each said conjugated diene contains 4 to 6 carbon atoms, said monomers being added in a ratio of 55 to 95, preferably of 60 to 90, and most preferably of 65 to 85 weight percent, monovinylaromatic monomer to 5 to 45, preferably of 10 to 40, and most preferably of 15 to 35, weight percent conjugated diene monomer thus giving a resinous tapered block copolymer, wherein said monovinylaromatic monomer charged in step (a) is from 27 to 80, preferably from 40 to 67, and most preferably from 47 to 60, each weight percent of the total monovinylaromatic monomer charged in said method, and wherein said monovinylaromatic monomer charged in step (b) is from 5 to 21, preferably from 8 to 19 and most preferably from 11 to 16 weight percent of the total monovinylaromatic monomer charged in said method. The invention also relates to a polymodal coupled resinous block copolymer of a monovinyl aromatic compound and a conjugated diene, said copolymer having polymer chains which result from coupling a mixture comprising S-S-B'S-B/S-Li and S-B/S-B/S-Li wherein S = monovinylaromatic block B = conjugated diene block B/S = tapered block Li = living polymer site or coupling site, wherein said mixture is obtained by the above process. Detailed Description of the Invention We have discovered monovinylaromatic/conjugated diene tapered block copolymers which can be used neat or blended with polymers of styrene to produce resins which can be formed into articles with low blueness and advantageous impact properties and/or ductility. The polymers of this invention are characterized as resinous non-rubbery block copolymers of at least one conjugated diene with at least one monovinylarene, having two random-tapered blocks and are prepared so that, when the choice of coupling agent permits. at least a portion of the final product is of a branched coupled character. The polymers prepared according to this invention contain from 55 to 95, preferably from 60 to 90, more preferably from 65 to 85, weight percent of copolymerized monovinyl aromatic monomer based on the weight of total monomers employed. Correspondingly, the inventive copolymers contain from 45 to 5, preferably from 40 to 10, and more preferably from 35 to 15 weight percent copolymerized conjugated diene monomer based on the total weight of monomers in the copolymer. The coupled portions of the resinous polymodal block copolymers of this invention have terminal polymonovinylarene blocks on the extending arms of each linear or radial copolymer molecule, and further contain two internal tapered block of monovinylarene and conjugated diene. The resinous copolymeric polymodal products also contain portions of linear uncoupled block copolymers of poly(monovinylarene)-poly(conjugated diene); the linear uncoupled block copolymer content is considered to be an important portion of the resinous product with respect to its overall properties. Components The process of this invention can be carried out using as an initiator any of the organomonoalkali metal compounds of the formula RM wherein R is an alkyl, cycloalkyl or arylcarbanion containing 4 to 8 carbon atoms and M is an alkali metal cation. Mixtures of organoalkali metal compounds can be used. The presently preferred initiators are alkylmonolithium compounds, especially n-butyllithium or sec-butyllithium. The conjugated diene monomers which can be used contain 4 to 6 carbon atoms and include 1,3-butadiene. 2-methyl-1,3-butadiene, 2-ethyl-1,3-butadiene: 2,3-dimethyl-1,3-butadiene and 1,3-pentadiene and mixtures thereof Each of the charges containing conjugated diene in the same sequence of charges may be the same, but is not necessarily the same, conjugated diene monomer or mixture of conjugated diene monomers. The presently preferred conjugated diene monomer is 1,3-butadiene. The monovinylaromatic monomers which can be used contain 8 to 12 carbon atoms and include styrene, alpha-methylstyrene, 4-methylstyrene, 3-methylstyrene, 2-methylstyrene, 4-ethylstyrene, 3-ethylstyrene, 2-ethylstyrene, 4-tert-butylstyrene, 2,4-dimethylstyrene and condensed aromatics such as vinyl napthalene and mixtures thereof. Each of the charges containing monovinylaromatic monomer in the same sequence of charges may be the same, but is not necessarily the same, monovinylaromatic monomer or mixture of monovinylaromatic monomers. The presently preferred monovinylaromatic monomer is styrene. Compounds employed as randomizers according to the invention are ethers, thioethers (sulfides) and tertiary amines. It is usually preferred to use ethers and sulfides in which the radicals attached to the oxygen or sulfur atoms are hydrocarbon radicals. Specific examples of such polar materials include dimethyl ether. diethyl ether, ethyl methyl ether, ethyl propyl ether, di-n-propyl ether, di-n-octyl ether, anisole, dioxane, 1,2-dimethoxyethane, dibenzyl ether, diphenyl ether, 1,2-dimethoxybenzene, tetramethylene oxide (tetrahydrofuran), dimethyl sulfide, diethyl sulfide, di-n-propyl sulfide, di-n-butyl sulfide. methyl ethyl sulfide, dimethylethylamine, tri-n-propylamine, tri-n-butylamine, trimethylamine, triethylamine, tetramethylethylenediamine, tetraethylethylenediamine, N,N-di-methylaniline, N-methyl-N-ethylaniline and N-methylmorpholine. It is to be understood also that mixtures of these polar compounds can be employed in the practice of the present invention. The polar compounds are generally used in admixture with the hydrocarbon diluent. Presently preferred are either tetrahydrofuran or diethyl ether. Among the suitable coupling agents are the di- or multivinylaromatic compounds, di- or multiepoxides, di- or multiisocyanates, di- or multiimines, di- or multialdehydes, di- or multiketones, alkoxytin compounds, di- or multihalides, particularly silicon halides and halosilanes, mono-, di-, or multianhydrides, mono-, di-, or multiesters, preferably the esters of monoalcohols with polycarboxylic acids, diesters which are esters of monohydric alcohols with dicarboxylic acids, lactones, monobasic acids with polyalcohols such as glycerol, including compounds containing two or more groups and mixtures of two or more compounds. Examples of suitable vinylaromatic coupling agents include, but are not limited to, divinylbenzene, 1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl and p-diisopropenylbenzene. Of these, the divinylaromatic hydrocarbons are preferred, particularly divinylbenzene in either its ortho, meta, or para isomer. Commercial divinylbenzene which is a mixture of the three isomers and other compounds is satisfactory. Epoxidized hydrocarbon polymers such as epoxidized liquid polybutadiene and epoxy compounds such as 1,2; 5,6: 9,10-triepoxydecane can be used as coupling agents. Organoalkyl phosphites and arylalkyl phosphites are considered useful as coupling agents in this invention. Examples of suitable multiisocyanate coupling agents include benzene-1,2,4-triisocyanate and naphthalene-1,2.5,7-tetraisocyanate. Commercially available products known as PAPI-1, a polyarylpolyisocyanate having an average of 3 isocyanate groups per molecule and an average molecular weight of about 380 are suitable. The multiimines, also known as multiaziridinyl compounds, such as those containing 3 or more aziridine rings per molecule, are useful as coupling agents. Other compounds useful as coupling agents include tetravinyl silane, trivinyl phosphine, the triaziridinyl phosphine oxides or sulfides such as tri(1-aziridinyl)phosphine oxide tri(2-methyl-1-aziridinyl)phosphine oxide and tri(2-ethyl-3-decyl-1-aziridinyl)phosphine) sulfide. The multialdehyde coupling agents are represented by compounds such as 1,6,7-naphthalenetricarboxyaldehyde, 1,7,9-anthracenetricarboxyaldehyde, 1,3,5-pentanetricarboxyaldehyde, and similar multialdehyde-containingaliphatic and aromatic compounds. The multiketones are represented by compounds such as 1,6,9,10-anthracenetetrone and 2.3-diacetonylcyclohexanone. Examples of the multianhydrides include pyromellitic dianhydride and styrene-maleic anhydride copolymers. Examples of the multiesters include diethyladipate, triethylcitrate, and 1,3,5-benzenetricarboxylic acid triethyl ester. Among the multihalide coupling agents are the silicon tetrahalides such as silicon tetrachloride silicon tetrabromide. and silicon tetraiodide: the trihalosilanes such as trichlorosilane, trichloroethylsilane and tribromobenzylsilane; and the multihalogen-substituted hydrocarbons. such as 1,3,5-tri(bromomethyl)benzene and 2,5,6,9-tetrachloro-3.7-decadiene, in which the halogen is attached to a carbon atom which is alpha to an activating group such as an ether linkage, a carbonyl group, or a carbon-to-carbon double bond. Substituents inert with respect to lithium atoms in the terminally reactive polymer can also be present in the active halogen-containing compounds. Alternatively, other suitable reactive groups different from the halogens as described above can be present. Other metal multihalides, particularly those of tin, lead, or germanium, can be employed as coupling and branching agents. Silicon or other metal multialkoxides, such as silicon tetraethoxide, are also suitable coupling agents. Examples of compounds containing more than one type of functional group include 1,3-dichloro-2-propanone, 2,2-dibromo-3-decanone, 2,4-dibromo-3-pentanone, 1,2: 4,5-diepoxy-3-pentanone 1,2: 4,5-diepoxy-3-hexanone, 1,2: 11,12-diepoxy-8-pentadecanone and 1,3: 18,19-diepoxy-7,14-eicosanedione. Useful multifunctional coupling agents include epoxidized vegetable oils such as epoxidized soybean oil, epoxidized linseed oil and the like or mixtures thereof. The presently preferred coupling agent is epoxidized vegetable oil. Presently preferred is epoxidized soybean oil. Process The unique polymodal tapered block character of the polymer and low blueness and good impact strength and/or ductility of articles made from the polymer or blends of the polymer of this invention are produced by the unique sequences of two separate initial charges of monovinylaromatic monomer and initiator, followed by a separate charge of a mixture of monovinylaromatic monomer and conjugated diene, a second separate charge of a mixture of monovinyl aromatic and conjugated diene monomers and a subsequent coupling step. In this invention the first initiator charge produces active living monovinyl aromatic component polymer blocks with alkali metal atoms (from the initiator) on at least one end to form active reaction sites. Each subsequent monomer charge adds monomer to the living polymer chain at the alkali metal reaction site. At each stage of charging, polymerization is allowed to continue until essentially no free monomer is present. With each subsequent charge which includes initiator a new polymer-alkali metal species will be produced. and each subsequent monomer charge has an opportunity for polymerization of part of the charge with each of the existing polymer-alkali metal species. Each of the active living polymer chains will have terminal monovinyl aromatic blocks on both ends after polymerization of each monomer charge containing monovinyl aromatic. Even when mixtures of monovinyl aromatic monomer and conjugated diene are charged, the polymer chains will have terminal monovinyl aromatic blocks on both ends. After virtually complete polymerization of the final monomer charge, the active living linear block copolymers are charged with a difunctional or polyfunctional coupling agent to allow coupling of each of the living species with each of the other living species or with others of the same living species to form the desired polymodal tapered block copolymers. If the coupling agent is not 100 percent efficient and/or if less or more than a stoichiometric amount of coupling agent is used, there can be some uncoupled terminated polymer chains of each of the species in the final reaction mixture. Use of difunctional coupling agents will produce predominantly linear polymer chains. Depending upon functionality, various degrees and kinds of branching may be accomplished with polyfunctional coupling agents. Variations in the amount of a particular polyfunctional coupling agent also can be used to manipulate the degree and kind or branching at the coupling sites. The charging sequences of this invention and the resulting polymers at each stage are exemplified using a selected monovinylaromatic monomer, conjugated diene and polyfunctional coupling agent in the following Table 1. Invention Charging Sequence Charge Contents of Charge Resulting Polymer Chains (a) randomizer, initiator 1 and styrene 1 S 1 -Li 1 (b) initiator 2 and styrene 2 S 1 -S 2 -Li 1 S 2 -Li 2 (c) butadiene 1 and styrene 3 S 1 -S 2 -B 1 /S 3 -Li 1 S 2 -B 1 /S 3 -Li 2 (d) butadiene 2 and styrene 4 S 1 -S 2 -B 1 /S 3 -B 2 /S 4 -Li 1 S 2 -B 1 /S 3 -B 2 /S 4 -Li 2 (e) coupling agent polymodal tapered block copolymers with styrene terminal blocks where S = styrene B = butadiene B/S = tapered block Li = residue from a monoalkali metal initiator remaining on the end of the polymerization chain or reaction site prior to termination or coupling. subscripts = designation of the numerical order in which that particular component was charged or formed. The randomizer is usually added with the diluent initially charged to the reactor. Each of the charges which has two monomers may be either a mixture of the two monomers or simultaneous charging of two separate monomers. As can be seen from the intermediate products listed in the charging sequence table above, there are at least two distinct species of polymer chains before coupling. Thus, polymodal tapered block copolymers comprising relatively high and low molecular weight species can be produced. Tapered blocks in each of the growing polymer chains are produced by simultaneously charging with at least two monomers as shown in the preceeding tables of the inventive charging sequences. The randomizer regulates tapering or random polymerization of the monovinylaromatic monomer and the conjugated diene in a mixed monomer charge. Choice of randomizer can be used to manipulate the degree of taper in blocks resulting from charges of mixtures of monomers. The taper is a graduation from conjugated diene rich chain to monovinylaromatic rich chain. That is, the diene enters into the chain faster than the monovinyl substituted aromatic; therefore, when both the monovinylaromatic monomer and the conjugated diene are present, the block tapers gradually from an essentially polybutadiene block to an essentially monovinyl substituted aromatic polymer block. The weight ratio of monovinyl substituted aromatic monomer to conjugated diene monomer in each of the tapered blocks is 1:0.63 to 1:2, preferably 1:0.67 to 1:1.8, and more preferably 1:0.8 to 1:1.5. The weight ratios of monovinyl substituted aromatic monomer to conjugated diene monomer in each of the tapered blocks in the same polymer chain do not have to be the same. See Example III. Generally each of the two tapered blocks made in steps (c) and (d) can be of equal size : however, actual sizes of the two tapered blocks can vary within the same copolymer depending upon the amounts of monomers charged in each of the third and fourth monomer charges. See run 13 in Example III. Prior to coupling, all of the living polymer chains have monovinylaromatic terminal blocks on one end because of the initial monovinylaromatic charge (a) and charge (b) made with initiator: the living ends of the chains will have tapered blocks prior to coupling because of the charge containing both monovinylaromatic and conjugated diene monomers made next preceeding the coupling step. In addition to the sequence of additions of the monomers and of the initiator, it is important to control the amount of each monomer and initiator addition at each stage or increment so that a suitable proportion of block sizes and proportion of polymodality is obtained. Generally in a presently preferred embodiment of this invention, if a polymer which is about 75 weight percent monovinylaromatic, based on total monomer weight, is used. 27 to 80 weight percent of the total monovinylaromatic monomer is charged in step (a), 5 to 21 weight percent of the total monovinylaromatic monomer is charged in step (b), 3 to 19 weight percent of the total monovinylaromatic monomer is charged in step (c), and 13 to 36 weight percent of the total monovinylaromatic monomer is charged in step (d). Generally more preferably, 40 to 67 weight percent of the total monovinylaromatic monomer is charged in step (a), 8 to 19 weight percent of the total monovinylaromatic monomer is charged in step (b), 5 to 16 weight percent of the total monovinylaromatic monomer is charged in step (c), and 16 to 32 weight percent of the total monovinylaromatic monomer is charged in step (d). Generally presently most preferably, 47 to 60 weight percent of the total monovinylaromatic monomer is charged in step (a). 11 to 16 weight percent of the total monovinylaromatic monomer is charged in step (b), 8 to 13 weight percent of the total monovinylaromatic monomer is charged in step (c), and 20 to 25 weight percent of the total monovinylaromatic monomer is charged in step (d). In the process of this invention it is feasible to stretch out over an interval of time the addition of one or more of the increments of initiator, thus spreading (increasing) further the polymodality of the resulting product upon coupling. The polymerization process is carried out in a hydrocarbon diluent at any suitable temperature in a range of -10° to 150°C, preferably in the range of 0° to 110°C, at pressures sufficient to maintain the reaction mixture substantially in the liquid phase. Temperatures and pressures will peak during polymerization of each monomer charge and then decrease when essentially no free monomer is left to react. Appropriate hydrocarbon diluents include linear and cycloparaffins such as butane, pentane, hexane, octane, cyclohexane, cyclopentane, and mixtures thereof. Presently preferred is cyclohexane. Generally the choice of hydrocarbon or hydrocarbon mixture and the temperature is such that the resulting polymer is in solution. Small amounts of polar compounds are used to improve the effectiveness of alkylmonoalkali metal initiators such as n-butyllithium: dissociation of the alkylmonoalkali metal initiators affects the rate of initiation and polymerization. The polar compounds also effect partial randomization of the vinylarene/conjugated diene so as to increase the random portion of the tapered block. The polar compounds are generally used In admixture with the hydrocarbon diluent. The amounts of polar compounds used as randomizers and promoters of effectiveness of initiators in this invention will vary according to the reactivity and effectiveness of the particular randomizer used. For example, 1,2-dimethoxyethane, tetramethylethylenediamine and 1,2-dimethoxybenzene are much more efficient randomizers than most of the others listed above when used with the particular initiators and monomers used in the invention runs described below. However, tetrahydrofuran is often used because the reaction will go nearer to completion in a shorter time after the initial reaction in the monomer rich environment. Also, there are dramatic variations in the amounts of each of these most efficient randomizers which will be needed. The amounts of polar compounds used as randomizers will also vary according to the desired molecular structure of the portions of tapered blocks which result from conjugated diene addition. For example, when using tetrahydrofuran, and it is desirable that the tapered blocks of the polymer have nearly equal amounts of 1,4 addition of butadiene and 1,2 addition of butadiene or only a little more 1,4 addition than 1,2 addition of butadiene, larger amounts of the tetrahydrofuran would be used than when more 1,4 addition is desired. When polymers with higher vinyl character resulting from 1,2 addition in excess of 1,4 addition are desired, then the useful amounts of tetrahydrofuran needed would be considerably larger. However, use of too much randomizer can result in excessive polymer-lithium termination during polymerization and/or poor stability of the polymer and/or undesired side reactions. depending upon choice of randomizer. Use of too little randomizer would result in inefficient initiator use, compositional variations and broader molecular weight distribution. The initial monovinylaromatic charge is made with the randomizer present for the additional effect of causing the monovinylaromatic component resulting from each initiator charge to be of relatively narrow molecular weight distribution. In the process of this invention, by varying the amounts of initiator in each of the two charges having initiator, the differences in molecular weights of the monovinylaromitic components resulting from each of the two charges can be increased In the process of the invention, amounts of initiator employed are those which will produce resins with desirable melt flow which can be used in blends to make articles with a good balance of properties including minimal blueness, and good impact strength and/or ductility. Presently preferred when making invention polymers to be used in blends are amounts of initiator in each of the two initiator charges sufficient to obtain a block copolymer having a melt flow in the range from 2 to 50 g/10 minutes, more preferably from 4 to 30 g/10 minutes. and most preferably from 7 to 20 g/10 minutes, as determined by ASTM D1238-73. condition 200°C/5.0 kg. The amounts of initiator contemplated as useful in each of the two charges having initiator are shown in Tables 3 and 4. Use of too small an amount of initiator would result in high molecular weight polymers. Conversely, use of too large an amount of initiator would result in polymers having short chain polymeric species and low molecular weight. The weight ratio of the amounts of initiator in each of the charges having initiator can be represented as 1:0.25-300. Li 1 : Li 2 :: 1 : 0.25-300 wherein Li 1 = initiator in the first charge Li 2 = initiator in the second charge More preferably for most applications, the amount of initiator in the second charge of initiator is from 0.3 to 10 times, based on weight, as much as the amount of initiator in the first initiator charge. Generally, presently most preferably, the amounts of initiators are selected such that the amount in the second charge is only slightly smaller. equal to. or larger than that employed in the first charge. Varying the weight ratios of amounts of the initiator charges will result in variations of the proportionate amounts of species present in the copolymer. Other factors affecting the proportionate amounts of species present in the copolymer include presence of impurities and/or scavengers in the reactor, effectiveness of the polar randomizer as a promoter and choice of coupling agent(s). The polymerization is carried out in substantial absence of oxygen and water preferably under an inert gas atmosphere. Prior to the coupling step, the reaction mass contains a very high percentage of molecules (polymer chains) in which an alkali metal cation is positioned at one end of each polymer chain. Impurities in the feed such as water or alcohol reduce the amounts of monoalkali metal polymer in the reaction mass. After essentially complete polymerization of the final charge added to the polymer, one or more suitable difunctional or polyfunctional coupling agents is added. As used here, the term "coupling" means the bringing together and joining, by means of one or more central coupling atoms or coupling moieties, two or more of the living monoalkali metal-terminated polymer chains. A wide variety of compounds for such purposes can be employed. Any effective amount of the coupling agent can be employed. While the amount is not believed to be particularly critical, a stoichiometric amount relative to the active polymer-alkali metal tends to promote maximum coupling as a generality. Presently preferred is an amount of coupling agent slightly greater than stoichiometric relative to the active polymer-alkali metal. However, less than stoichiometric amounts can be used for higher degrees of coupling where desired for particular products of broadened molecular weight distribution. Typically, in the process of this invention, the total amount of coupling agent is in the range of 0.005 to 10 phm (parts per 100 parts of total monomers employed in the polymerization). Preferred when most combinations of monomers and coupling agents are used to practice this invention is 0.2 to 0.6 phm of coupling agent, depending on amounts of initiator used. Presently most preferred is 0.3 to 0.5 phm, depending upon amounts of initiator used. Use of an amount of a reasonably highly efficient coupling agent within these ranges provides polymers with a moderately broad molecular weight distribution that has proven useful to custom molders. Use of an insufficient amount of coupling agent will result in less complete coupling of the living polymer chains or, depending upon choice or coupling agent, more branching; use of an excessive amount of coupling agent will also result in more uncoupled chains. At the conclusion of the coupling process, the coupled polymer may still contain bound polymeric alkali metal alkoxides depending on the type of coupling agent employed. The system is treated with an active compound such as water, alcohol, phenols, carbon dioxide or linear saturated aliphatic mono- and dicarboxylic acids to remove any remaining alkali metal from the copolymer chain. While the polymer product is still in solution, stabilization agents can be added. Additional stabilizers could be added during finishing before pelletizing. This treatment will provide oxidative stability for the polymer during processing and handling and subsequent long term use by the customer. Commonly used stabilization processes can use a combination of compounds which include, but are not limited to, a hindered phenol and an organophosphite, particular examples of which are octadecyl 3-(3',5'-di-t-butyl-4'-hydroxyphenyl) propionate and tris-nonylphenylphosphite. After stabilization, the hydrocarbon diluent is then flashed from the polymer solution to increase the solids content. The polymer cement, i.e., the polymer in the polymerization solvent, usually contains 10 to 40, more usually 20 to 35, weight percent solids, the balance solvent. Preferably, but not necessarily, the polymer cement is flashed to remove by evaporation a portion of the solvent so as to reduce the solvent content to a concentration of 0 to 50, more usually 0 to 10, weight percent (corresponding to a solids content of 100 to 50, more usually 100 to 90, weight percent). Flashing of the polymer cement may be followed by desolventizing extrusion with vacuum in commercial production or by other vacuuming processes to achieve consistent solvent content of less than 0.3 weight percent. The resinous copolymeric products can be, and normally are, compounded with anti-oxidants, anti-blocking agents, release agents and other additives, as known in the compounding arts. Typical charging sequences and useful ranges of amounts of the charges for the process of this invention are given in Table 2. After coupling at least the coupled and uncoupled polymeric species shown in Table 3 are present in the polymodal polymer of this invention. Polymeric Species Included S 1 -S 2 -B 1 /S 3 -B 2 /S 4 -x-S 4 /B 2 -S 3 /B 1 -S 2 -S 1 S 1 -S 2 -B 1 /S 3 -B 2 /S 4 -x-S 4 /B 2 -S 3 /B 1 -S 2 S 2 -B 1 /S 3 -B 2 /S 4 -x-S 4 /B 2 -S 3 /B 1 -S 2 S 2 -B 1 /S 3 -B 2 /S 4 S 1 -S 2 -B 1 /S 3 -B 2 /S 4 S = monovinylaromatic block B = conjugated diene block B/S = tapered block x = coupling moiety or coupling site subscripts = indications of the charges which were the source of the polymer blocks. Depending upon choice and amount of coupling agent or agents and whether coupling agents are charged as a mixture or incrementally. there can be present other polymeric species with varying degrees of branching. Blends The resinous polymodal copolymer products of this invention can be blended with other polymers such as acrylonitrile-butadiene-styrene copolymers (ABS), styrene-acrylonitrile copolymers (SAN), and other styrene copolymers. When less transparency is desired or transparency is not necessary, the invention copolymers can be blended with polyolefins and/or olefin copolymers. Blends of the invention polymers with polymers of styrene are particularly useful applications for the improved tapered block copolymers of this invention. Articles made from these blends have surprisingly low blueness and advantageous impact strength and/or ductility. For example, articles made from styrene homopolymer and either of the two embodiments of this invention typically have Hunter negative b blueness values of less than 20, most often less than 18. Thus, less of the invention copolymer is needed to achieve high impact strength, while retaining reasonably low blueness. in articles made from blends with styrene homopolymers than would be needed of other comparative polymodal monovinylaromatic-conjugated diene block copolymers. The presently preferred polymers of styrene employed in the blends of this invention are usually (a) homopolymers of styrene; or (b) copolymers of styrene as a major component with a minor amount of any other copolymerizable monovinyl aromatic compound other than styrene, such as alpha-methylstyrene, vinyltoluene or para-tertiary-butyl styrene. A minor amount of other monomers such as methyl acrylate, methyl methacrylate and acrylonitrile can be copolymerized with the styrene. The invention copolymers can be blended with styrene resins made in bulk polymerization. These resins are commonly prepared by heating styrene and any comonomer at temperatures in the range of 100° to 200°C with application of pressure, if necessary, to combine the monomers. The polymerization can also be carried out at lower temperatures by the addition of free-radical generating peroxidic catalysts such as benzoyl peroxide, acetyl peroxide and di-t-butyl peroxide. Alternatively, the polymerization can be carried out in suspension to yield a dry powder or in emulsion, usually resulting in a latex of polystyrene which can be coagulated to yield the solid powdery polystyrene. The polymerization can also be carried out in solution with precipitation of the product, if desired. Solvent can be removed by standard techniques such as steamstripping or solvent evaporation. High impact polystyrene (HIPS) also can be successfully employed in blends with the invention copolymers. Suitable high impact polystyrenes can be prepared by polymerizing styrene in the presence of an elastomer, typically polybutadiene rubber. In these resins the polystyrene forms the bulk of the resin throughout which the grafted rubber polystyrene particles are dispersed. The blends of this invention can be prepared by any suitable means including blending, tumbling and extrusion. Examples of these methods include, but are not limited to, dry mixing in the form of a powder or pellets, wet mixing in the form of a solution or slurry, and melt extrusion compounding. The polymers and any other ingredients or additives may be mechanically blended together in the desired proportions with the aid of any suitable mixing device conveniently used for mixing rubbers or plastics, such as, for example, a differential roll mill, a Banbury mixer, or an extruder. In these types of blending methods the polymers and any other components and additives used can be in any form, such as, for example, fluff, powder, granulate, pellet, solution, slurry, and/or emulsion. Any additive can be combined with the polymers according to any method known in the art. Examples of incorporation methods include, but are not limited to, dry mixing in the form of a powder and wet mixing in the form of a solution or slurry. Melt extrusion compounding can be carried out using any suitable method such as in single screw or twin screw extruders or other melt extruders at temperatures above the melting point or glass transition temperature of the polymers. The presently preferred method comprises blending the polymers in powder or granulate form and extruding the blend in sheet form to feed a thermoformer or direct feed to an injection or blow molder. In order to facilitate thorough mixing of the polymers and to develop the desired combination of physical properties, pellets are generally metered by loss-in-weight feeders or by screw feeders at a temperature low enough to avoid softening the pellets. The metered pellets are dropped into an extruder which melts and blends the components to provide a homogenous melt. Alternatively, solution blending methods known in the art may be used. The ranges of amounts of polymers useful in blends varies according to the properties and economics required. For example, when an invention copolymer is blended with a polymer of styrene, practical ranges include using amounts such as 10 to 70 weight percent polymer of styrene, more usually 20 to 65 weight percent polymer of styrene, and most preferably 30 to 60 weight percent polymer of styrene with the balance being one or more of the polymodal resinous copolymer products of this invention. For a more particular example, when the invention copolymers are blended with general purpose polystyrene, broad ranges include using amounts such as 10 to 70 weight percent polystyrene, more usually 20 to 65 weight percent polystyrene, and most preferably 30 to 60 weight percent polystyrene with the balance being one or more of the polymodal resinous copolymer products of this invention. Generally, use of too much of the invention copolymers in a blend would result in good properties but loss of economic advantage. Use of too little of the invention copolymers in a blend would result in loss of impact resistance. These blends can be economical ways of gaining the desirable attributes of both polystyrene and the invention polymers while maintaining low blueness in articles made from the blends. The compositions of this invention or blends thereof can be extruded, thermoformed, injection molded, blow molded, or made into films or sheets. Articles made from the compositions of this invention are transparent with low blueness, have good impact strength and have other physical properties within acceptable ranges for such applications as drinking cups, lids, bottles, other food containers, medical drainage units, shrink wrap and over-wrap. Articles made from blends of the copolymers of this invention can economically provide advantageous properties for similar applications. Test Procedures The blend blueness values were determined on a Hunter Lab colormeter Model D 25 using the Hunter Lab procedure. Blueness values are expressed as -b, where larger absolute numbers indicate more blueness. Other properties were tested using ASTM procedures as shown in Table 4. Test Procedures Used Property ASTM Method Flow rate, g/10 min Condition 200°C/5.0 kg D 1238-88 Haze, % D 1003-61 (1990) Transmittance, % D 1003-61 (1990) Shore D hardness D 2240-91 Tensile strength at yield and break, MPa D 638-91 at 50 mm/min Type I test specimens Elongation at yield and break, % D 638-91 at 50 mm/min Flexural modulus MPa D 790-86 Izod impact strength, notched, J/m D 256-88 Vicat softening point, °C D 1525-91 Total energy dart drop, J D 4272-85 Examples The following examples will describe in more detail the experimental process used and the polymodal tapered block copolymers with vinylarene terminal blocks obtained as a result of the process and blends made with the invention copolymers. These examples should be taken as illustrative and not restrictive. Styrene and butadiene were chosen as monomers to exemplify the invention, and randomizer, initiator, coupling agent and diluent appropriate for these monomers were used. Quantities of reagents are usually expressed in parts per hundred monomer (phm) based on the total weight of monovinylarene and conjugated diene employed. Example I To demonstrate the invention, three polymerization runs were carried out according to the general procedure described below, with the sequences and amounts of charges as shown in Table 5. Tapered butadiene/styrene blocks were formed in the third and fourth steps by charging a mixture of butadiene and styrene monomers. The monomer addition sequence was i, S 1 , i, S 2 , B 1 /S 3 , B 2 /S 4 followed by coupling; the monomer weight ratios were 40, 10, 12.5/12.5,12.5/12.5, respectively. The polymers were 75 percent styrene and 25 percent butadiene. In each of the three runs of this example the weight ratio of amount of initiator in the first step to the amount in the second step was kept constant at 1:1 with the absolute amount varied from 0.06 phm (run 8) to 0.055 phm (run 10). The copolymers produced in the three runs were designated invention copolymers 8, 9, and 10, and had melt flows of 14.3, 6.40 and 10.8 respectively. It is believed that lower melt flows are attributable to lower amounts of initiator. The polymerizations of invention runs 8, 9 and 10 are shown in Table 5. General procedure Styrene (from Stirling Chemical) and 1,3-butadiene (from Texas E1 Paso) were dried by passage over activated alumina (Kaiser A-201), and then copolymerized and coupled in a 4-stage process using n-butyllithium initiator (from Lithium Corporation of America). Polymerization runs were carried out under nitrogen in a stirred, jacketed, stainless steel 7.6-liter reactor employing essentially anhydrous reactants and conditions. The anhydrous mixtures were stirred continuously during the polymerization process. The cyclohexane diluent, which contained 0.04 phm tetrahydrofuran (THF) in each polymerization in this example, was preheated to about 50°C before monomers were charged to the reactor. The n-butyllithium was charged as a 2 weight percent solution in cyclohexane. In the polymerization step in which both butadiene and styrene were charged, they were charged simultaneously as a mixture. In the coupling step, the Vikoflex® 7170 coupling agent used was an epoxidized vegetable oil commercially available from Viking Chemical Company. In the terminating step, carbon dioxide from a pressurized container was admitted to provide about 0.4 phm carbon dioxide to the reactor. Water was also added In an amount slightly in stoichiometric excess of the initiator to separate the lithium residues from the polymer chains. The antioxidant mixture added in the stabilizing step contained a hindered phenol [octadecyl 3-(3',5'-di-t-butyl-4'-hydroxyphenyl) propionate, commercially available as Irganox® 1076 from Ciba-Geigy] and an organic phosphite (tris-nonylphenyl phosphite, available as TNPP from GE Specialty Chemicals). Each stabilizer was dissolved separately in cyclohexane and mixed together. Enough of the mixture was added to the reactor to provide 0.25 phm hindered phenol and 1 phm organic phosphite. In Runs 1 and 2 a microcrystalline wax (BE Square® 195) was also added as an anti-blocking agent. After each addition of monomer, initiator or additive, the feed lines were rinsed with approximately 10-20 phm cyclohexane solvent and cleared with a nitrogen sparge. Following the stabilization step, each copolymer solution was flashed at 178-180°C to remove a portion of the diluent. Substantially all of the remaining diluent was removed in a vacuum oven by drying at 90°C for one hour. The resulting polymer was chopped in a granulator into crumb size and then dried for an additional hour in a vacuum oven. Invention Runs Components Step 1 Run 8 Run 9 Run 10 Cyclohexane, phm 145 145 145 Tetrahydrofuran, phm 0.04 0.04 0.04 n-Butyllithium initiator, phm 0.06 0.05 0.055 Styrene, phm 40 40 40 Polymerization Time, min 12 12 12 Peak Polymerization Temperature, °C 85 82 84 Step 2 n-Butyllithium initiator, phm 0.06 0.05 0.055 Styrene, phm 10 10 10 Polymerization Time, min 12 12 12 Peak Polymerization Temperature, °C 69 69 70 Step 3 Butadiene, phm 12.5 12.5 12.5 Styrene. phm 12.5 12.5 12.5 Polymerization Time. min 16 16 16 Peak Polymerization Temperature, °C 84 83 84 Step 4 Butadiene, phm 12.5 12.5 12.5 Styrene, phm 12.5 12.5 12.5 Polymerization Time, min 16 16 16 Peak Polymerization Temperature ,°C 96 102 102 Step 5 (coupling) Vikoflex 7170, phm 0.4 0.4 0.4 Time, min 16 16 16 Temperature, °C 82 89 89 Step 6 (Terminating) Water, phm 0.2 0.2 0.2. Carbon Dioxide, phm 0.4 0.4 0.4 Time, min 25 25 25 Temperature, °C 80 82 82 Step 7 (Stabilizing) Stabilizer Mixture, phm 1.25 1.25 1.25 Antiblocking agent, phm 0.15 0.15 0.15 Time, min 5 5 5 Temperature, °C 80 n.a. 81 Recovered Resin Melt Flow, g/10 min 14.3 6.4 10.8 Mw/Mn, thousands 154/104 173/115 147/91 Heterogeneity Index 1.48 1.50 1.62 Example II A series of blends of double tapered block invention polymers with polystyrene was prepared to demonstrate the properties of articles made from the blends. Copolymers 8, 9, and 10 from Example I were blended as described below in 50:50 by weight mixtures with general purpose polystyrene Novacor ® 555 to form blends 8', 9', and 10'. Test specimens were injection molded from the blends as described below. The results are shown in Table 6. Blending and Specimen Production The blends were 50:50 by weight and were prepared by solution blending in cyclohexane in a reactor at 100°C with stirring for one hour. Each solution was flashed to remove the solvent and the polymer was dried, chopped in a granulator, and dried again. The dry polymer was processed on a roll mill and chopped again before injection molding on an Arburg model 90 injection molder with zones 1,2, 3, and nozzle at 210°C, 210°C,220°C, and 210°C, respectively, to make specimens to test for properties. For convenient reference, blends are designated as blends with the corresponding polymer number and a prime. For example, a blend prepared from polymer 8 and polystyrene is blend 8' and a blend prepared from polymer 9 and polystyrene is designated blend 9'. Physical Properties of Polymer Blends With Polystyrene Property Invention Blend 8' Invention Blend 9' Invention Blend 10' Styrene-butadiene copolymer 8 9 10 Blend melt flow, g/10 min 14.8 12.0 14.8 Haze, % 4.1 4.0 5.1 Hunter blueness, b -17.9 -18.5 -17.9 Shore D hardness 76 77 76 Notched Izod impact, J/m 14.9 17.1 19.2 Vicat softening, °C 89.3 90.5 90.5 Tensile Strength yield MPa 43.2 43.9 42.5 break MPa 28.6 28.7 28.7 Elongation yield, % 6.4 6.5 6.5 break, % 17.9 16.7 13.6 Flexural modulus, MPa 1988 1949 1986 Example III Three more invention copolymers were prepared on a larger scale to demonstrate further the effect of two butadiene/styrene tapered blocks in improving impact properties of articles made from blends of the invention copolymers with polystyrene. These polymerizations varied the size of the two tapered blocks and the weight ratios of initiator charges. The polymerizations were carried out in a 380 L reactor. The polymer polymerization recipes are shown in Table 7 Each polymer was prepared with the sequence i,S,i, S,B/S,B/S. Polymer 13 had monomer charge weight ratios of 40, 10, 6.9/7.8, 18.1/17.2. Polymer 14 had monomer charge weight ratios of 40, 10, 5.9/10, 19.1/14.4. Polymer 15 had monomer charge ratios of 40, 10, 5.9/10, 19. 1/14.4. Monomer charge ratios for polymers 14 and 15 do not add up to 100 phm because of charge irregularities; phm indicated above was that actually charged. The initiator weight ratios for steps 1:2 in polymerizations 13, 14, and 15 were 1:0.79, 1:0.92, and 1:0.89, respectively. Flow rates for the three polymers were 7.0, 11.5, and 13.7 g/10 min. Invention Runs Step 1 Run 13 Run 14 Run 15 Cyclohexane, phm 168.9 168.9 168.9 Tetrahydrofuran, phm .04 .04 0.04 n-Butyllithium initiator, phm 0.061 0.061 0.065 Styrene, phm 40 40 40 Cyclohexane, phm 1.1 1.1 1.1 Peak Polymerization Temperature, °C 83 83 83 Peak Polymerization 214 (31) 221 (32) 207 (30) Pressure, kPa (psi) Step 2 n-Butyllithium initiator, phm 0.048 0.056 0.058 Styrene, phm 10 10 10 Cyclohexane, phm 1.1 1.1 1.1 Peak Polymerization Temperature, °C 81 82 82 Peak Polymerization Pressure, kPa (psi) 242 (35) 255 (37) 235 (34) Step 3 Butadiene, phm 6.9 5.9 5.9 Styrene, phm 7.8 10 10 Cyclohexane, phm 1.1 1.1 1.1 Peak Polymerization Temperature, °C 110 109 109 Peak Polymerization Pressure, KPa (psi) 394 (57) 400 (58) 380 (55) Step 4 Butadiene, phm 18.1 19.1 19.1 Styrene, phm 17.2 14.4 14.4 Cyclohexane, phm 1.1 1.1 1.1 Peak Polymerization Temperature, °C 95 98 97 Peak Polymerization Pressure, kPa (psi) 359 (52) 373 (54) 345 (50) Polymer Analysis (prior to coupling) Mw/Mn, thousands (GPC area composition) Peak 1 104/99 (64) 100/95 (63) 95/90 (64) Peak 2 50/46 (36) 47/44 (37) 46/43 (36) Step 5 (Coupling) Vikoflex 7170, phm 0.4 0.4 0.4 Cyclohexane, phm 0.55 0.55 0.55 Temperature, °C 88 86 88 Pressure, kPa (psi) 345 (50) 359 (52) 331 (48) Step 6 (Terminating) Water, phm 0.2 0.2 0.2 Carbon Dioxide, phm 0.1 0.1 0.1 Temperature, °C 87 87 87 Pressure, kPa (psi) 345(50) 345(50) 345(50) Step 7 (Stabilizing) Stabilizer Mixture, phm 1.25 1.25 1.25 Antiblocking agent, phm 0.3 0.3 0.3 Recovered Resin Melt Flow, g/10 min 7.0 11.5 13.7 Example IV Polymers from Example III were blended with Novacor® 555 general purpose polystyrene to produce blends 13', 14', and 15' for evaluation. The resulting blend was extruded on an 8.9 cm diameter extruder and sheet line. All blends were 50:50 by weight. A 0.51 mm die gap opening was used for the operation of a 0.38 mm thick extruded sheet for evaluation. The extruded sheet samples were evaluated and the results are shown in Table 8. The fold test involves bending the sheet on itself in the machine direction (MD) to induce a transverse directional (TD) break. The sheet is folded at two different rates to allow differentiation between brittle sheet samples. In audition, the behavior of the sheet in response to a tear in machine and transverse directions was determined. Sheets made from invention blends 13', 14' and 15' all had low blueness. Sheets made from blend 13' had the highest total energy dart drop value of sheets from this set of blends. Sheets made from blends 14' and 15' had low total energy dart drop values. Physical Properties of Polymer Blends With Polystyrene Property Invention Blend 13' Invention Blend 14' Invention Blend 15' Styrene-butadiene copolymer 13 14 15 Copolymer: Styrene blend ratio 50:50 50:50 50:50 Hunter blueness b -8.0 -10.0 -10.1 Haze, % 2.46 2.48 2.97 Transmission, % 89.6 89.3 89.2 Total energy dart drop, J 3.17 0.80 1.11 Transverse direction Fold (fast/slow) D/D B/D B/D Machine direction Tear D D D Transverase direction Tear D D D These results show that the copolymers of the invention can be used in blends with polymers of styrene to produce resins from which can be made articles having low blueness.

Claims: A method of preparing a polymer under solution polymerization conditions in a reaction zone consisting of: (a) charging a monovinylaromatic monomer and an initiator in the presence of a randomizer; which is an ether, thioether, or a tertiary amine; thereafter (b) charging an initiator and a monovinylaromatic monomer; thereafter (c) charging a mixture of a monovinylaromatic monomer and a conjugated diene monomer; thereafter (d) charging a mixture of a monovinylaromatic monomer and a conjugated diene monomer; and thereafter (e) charging the reaction mixture with a coupling agent, wherein in steps (a) and (b) 0.005 - 1.5 parts by weight per 100 parts by weight of total monomers each of initiator, are employed, wherein each said monovinylaromatic monomer contains 8 to 12 carbon atoms and each said conjugated diene contains 4 to 6 carbon atoms, said monomers being added in a ratio of 55 to 95, preferably of 60 to 90, and most preferably of 65 to 85 weight percent, monovinylaromatic monomer to 5 to 45, preferably of 10 to 40, and most preferably of 15 to 35, weight percent conjugated diene monomer, thus giving a resinous tapered block copolymer, wherein said monovinylaromatic monomer charged in step (a) is from 27 to 80, preferably from 40 to 67, and most preferably from 47 to 60, each weight percent of the total monovinylaromatic monomer charged in said method, and wherein said monovinylaromatic monomer charged in step (b) is from 5 to 21, preferably from 8 to 19 and most preferably from 11 to 16, weight percent of the total monovinylaromatic monomer charged in said method, wherein in each of steps (a) to (d) polymerization is conducted until essentially no free monomer is present. The method of claim 1, wherein the weight ratio of the amounts of said monovinylaromatic monomer and said conjugated diene monomer in each of steps (c) and (d) is within the range of 1:0.63 to 1:2, preferably of 1:0.67 to 1:1.8, and most preferably of 1:0.8 to 1:1.5. The method of claim 1 or 2, wherein said monovinylaromatic monomer charged in step (c) is from 3 to 19, preferably from 5 to 16, and most preferably from 8 to 13, weight percent of the total monovinylaromatic monomer charged in said method. The method of any of claims 1 to 3, wherein said monovinylaromatic monomer charged in step (d) is from 13 to 36, preferably from 16 to 32, and most preferably from 20 to 25, weight percent of the total monovinylaromatic monomer charged in said method. The method of any of the preceding claims, wherein said initiator in steps (a) and (b) is charged in amounts effective to provide polymer having a melt flow in the range from 2 to 50 g/10 min. The method of any of the preceding claims, wherein said initiator in step (b) is present in an amount at least equal to the amount of said initiator in step (a), preferably wherein the amount of initiator in step (b) is from 0.25 to 300 times as much, most preferably from 0.3 to 10 times as much, each based on weight, as the amount of initiator in step (a). The method of any of the preceding claims, wherein each said conjugated diene monomer is 1,3-butadiene. each said monovinylaromatic monomer is styrene, said organomonoalkali metal initiator is n-butyllithium, said randomizer is one chosen from tetrahydrofuran and diethyl ether, and said coupling agent is epoxidized vegetable oil. The method of any of the preceding claims, wherein polymerization is carried out in a hydrocarbon diluent: wherein said polymerization is carried out in a substantial absence of oxygen and water at temperatures ranging from -10 to 150 °C; wherein, after said coupling agent reacts with products of said polymerization, the system is treated with a terminating agent and a stabilizer and wherein, after termination with said terminating agents, any remaining hydrocarbon diluent is flashed off. A polymodal coupled resinous block copolymer of a monovinyl aromatic compound and a conjugated diene said copolymer having polymer chains which result from coupling a mixture comprising S-S-B/S-B/S-Li and S-B/S-B/S-Li wherein S = monovinylaromatic block B = conjugated diene block B/S = tapered block Li = living polymer site or coupling site. wherein said mixture is obtainable by the process of claim 3. The copolymer of claim 9 having from 55 to 95, preferably from 60 to 90, and most preferably from 65 to 85, weight percent polymerized monovinylaromatic monomer, based on total weight of monomers in said copolymer. Articles made from the copolymer of claim 9 or 10. Blends of a polymer of styrene and the copolymer of claim 9 or 10. The blend of claim 12, wherein said polymer of styrene is present in an amount in the range from 10 to 70, preferably from 20 to 65, and most preferably from 30 to 60, weight percent, based on total weight of said blend. Articles made from the blend of claim 12 or 13.

Inventors: FARRAR JR RALPH C, MOCZYGEMBA GEORGE A, STACY NATHAN E, TREPKA WILLIAM J, FARRAR JR., RALPH C., MOCZYGEMBA, GEORGE A., STACY, NATHAN E., TREPKA, WILLIAM J.
Applicants: PHILLIPS PETROLEUM CO, PHILLIPS PETROLEUM COMPANY
Application year: 1994
Main classification: C08F297
IPC codes: C08L25, C08G81, C08F297, C08L53, C08F8